1. Field of the Invention
The present invention generally relates to a lens holding structure, a method for assembling the lens holding structure, and a method for determining a dimension of the lens holding structure, and particularly relates to the lens holding structure in which the lens is held inside a lens frame by caulking, to the assembling method by caulking, to the lens holding structure in which an optical axis of the lens and a center of the lens frame can be aligned to each other, and to the method for determining the dimension of the lens holding structure.
2. Description of the Related Arts
A lens holding structure 1a and a lens holding structure 1b, as shown in the sectional views of FIGS. 1 and 2, have been conventionally used, in which a plurality of lenses 4 and 6 are inserted into and are mounted on a lens frame 2 in the same axial direction 5 that is shown by an arrow 5 in the figures. In the lens holding structure 1a or 1b, the first lens 4 is first mounted on the lens frame 2 by inserting it thereinto, and then the second lens 6 is mounted on the lens frame 2 by inserting it thereinto, both in the same direction 5.
After inserting the first lens 4 inside the lens frame 2 in the direction 5, the first lens 4 is fixed to the lens frame 2 by using a C-ring 8, engaging a downstream peripheral edge of the first lens 4 relative to the direction 5, as shown in FIG. 1, or by using an adhesive 9, locating on a side of a downstream peripheral edge of the first lens 4 relative to the direction 5, as shown in FIG. 2. or by using an unshown screw ring, etc.
Next, the second lens 6 is inserted inside the lens frame 2 in the same axial direction 5, and the second lens 6 is fixed to the lens frame 2 in such a way that a deformable caulking portion 3 which extends around a lens inserting peripheral edge of the lens frame 2 is thermally deformed by a heated jig 7. That is, the deformable caulking portion 3 is pressed against a downstream peripheral edge of the second lens 6 in the axial direction 5 so that the deformed caulking portion 3a presses the second lens 6 against a stepped portion, located on an upstream side relative to the deformable caulking portion 3a, of the lens frame 2.
In this manner, the second lens 6 is fixed to the lens frame 2 by caulking the deformable caulking portion 3a against the downstream peripheral edge of the second lens 6, with a high operation efficiency.
However, in respect of the first lens 4 which is first installed inside the lens frame 2, it is not practicable to fix the first lens 4 to the lens frame 2 by caulking. In order to fix the first lens 4 to the lens frame 2 by caulking, it is necessary to form a caulking portion, which is pressed against the first lens 4 along the axial direction 5 of the lens frame 2 by a heated jig as mentioned above, on an inner surface around an intermediate position relative to the axial direction of the lens frame 2.
With the construction, however, the outer diameter of the lens frame 2 cannot help but be larger, and at the same time, the outer diameter of the second lens 6 is required to be larger.
Namely, the first lens 4 which is first assembled inside the lens frame 2, before the second lens 6 is assembled therein, cannot be fixed to the lens frame 2 by caulking, without enlarging the lens frame 2 in its radial direction.
Meanwhile, in case that the lens holding structure in which a lens is held by a lens frame thereof for holding the lens, is installed inside an optical device, if the lens has a higher sensitivity in error, i.e. if the error of the lens seriously affects the performance of the optical device as in the aberration thereof, the lens is fixed to the lens frame with an optical axis of the lens being aligned relative to a center of the lens frame in order to enhance the accuracy in position of the lens with respect to the lens frame.
That is, as shown in a perspective view of FIG. 10 and a sectional view of FIG. 11, the lens frame 120 of the lens holding structure 101 has a projection 128 which projects inward and has a contact surface 126 with which a peripheral surface 114 on one side of the lens 110 contacts so as to position the lens 110 in the optical direction, and has an inner peripheral surface 124 with an inner diameter .o slashed.1 that is greater than an outer diameter .o slashed.2 of a lens 110. In the arrangement, both the inner peripheral surface 124 and the contact surface 126 constitute a lens holding part 122, of the lens frame 120, for holding the lens 110.
The lens 110 is fixed to the lens frame 120 with the above construction as follows.
That is, first, the lens 110 is inserted to the side of the lens holding part 122 of the lens frame 120, and the lens 110 is put on the contact surface 126 of the lens holding part 122.
Next, the lens 110 is properly moved relative to the contact surface 126 thereof in the radial direction of the lens 110 by using a lens pushing rod 130 as shown in FIG. 10, so that the lens 110 is positioned relative to the lens frame 120 with a condition in which a center of the lens frame 120 is aligned to an optical axis of the lens 110.
Next, using an adhesive applicator 132, an adhesive is filled between the lens holding part 122 and the lens 110 in order to fix the lens 110 to the lens holding part 122 of the lens frame 120, with the lens 110 being held in position relative to the lens holding part 122.
In the arrangement, there is a clearance or allowance in alignment (.o slashed.1-.o slashed.2) between the inner peripheral surface 124 of the lens frame 120 and the outer peripheral surface of the lens 110. The allowance in alignment (.o slashed.1-.o slashed.2) is the so-called "alignment allowance" therebetween. Within the alignment allowance, it is possible to adjust the lens 110 relative to the lens holding part 122 of the lens frame 120 so as to realize the alignment therebetween.
Generally, in case that the lens 110 is made of glass, it is manufactured, for example, by fusing, cutting and/or pressing the lens material so that the outer configuration thereof is roughly processed; then by grinding, lapping and/or polishing both surfaces 112 and 114 of the lens 110; and then by performing a centering of the lens. The "centering" is a work for finding out an optical axis, which connects a center of one curved surface of the lens and a center of the other curved surface thereof with each other, of the lens, and for abrading and rounding the lens into a predetermined peripheral configuration about the optical axis thereof so as not to be decentered from the optical axis.
The centering work of the lens 110 is carried out as follows.
As shown in a sectional view of FIG. 12, first, both surfaces 112 and 114 of the lens 110 are held or pinched by a pair of holders 102 and 103.
Next, the lens 110 is rotated about its rotation axis 105 while the peripheral surface 116 of the lens 110 is abraded or ground. Through this step, the difference between the axial lengths L1 and L2 on the peripheral surface 116 of the lens 110 is eliminated so that the optical axis 104 of the lens 110 is made to come to a center of the lens 110 with the optical axis 104 thereof being coincident with the rotation axis 105 of the lens 110.
However, generally, there occurs a decentering of the lens to some extent in case that the lens is a mass-produced lens. That is, the optical axis 104 of the lens 110 and the rotation axis 105 thereof do not accurately correspond to each other, and the optical axis 104 of the lens 110 and the rotation axis 105 thereof are spaced from each other by a distance .delta., as shown in FIG. 12.
In case that the decentering amount .delta. is larger than the alignment allowance (.o slashed.1-.o slashed.2), it is not possible to align the optical axis of the lens 110 with the center of the lens frame 120.
On the other hand, in case that the decentering amount .delta. is smaller than the alignment allowance (.o slashed.1-.o slashed.2), it is possible to align the optical axis of the lens 110 with the center of the lens frame 120. In this case, however, the inner diameter .o slashed.1 of the inner peripheral surface 124 of the lens holding part 122 becomes unnecessarily larger than is required. In other words, the size or dimension of the lens frame 120 in the radial direction is unnecessarily large, and this prevents a miniaturization of the lens holding structure and thus any apparatus in which the lens holding structure is used.
It is possible to reduce the decentering amount .delta. of the lens 110, by carefully performing the centering work of the lens 110. Generally speaking, in case that the lens is mass-produced, there is a tendency in which there is a variation in the decentering amount .delta. to a certain extent. Therefore, conventionally, in order to able to align the lens 110 relative to the lens frame 120 in view of the variation in the decentering amount .delta., the alignment allowance (.o slashed.1-.o slashed.2) has been selected experimentally or by trial and error; however, the alignment allowance (.o slashed.1-.o slashed.2) is liable to be too large which in turn leads to an unnecessarily large lens holding structure, and thus any apparatus or device in which the lens holding structure is used.